The force of muscle contraction is provided by the interaction of two proteins, myosin and actin, with ATP. My proposed research will be concerned with discovering the molecular mechanisms by which this interaction transduces the energy stored in ATP into mechanical thrust. Specifically I will investigate the role of the bound nucleotide of actin and role of the duplex nature of myosin. Although it has been the center of a number of studies the role of the actin nucleotide remains unknown. I will use an unsplittable analog of ATP, which I have recently shown can be incorporated into the F-actin structure, to search for some interaction of actin which is dependent upon the nature of the bound nucleotide. The question of whether both heads of myosin are necessary for its interaction with actin has been studied with conflicting results. I will extend these studies to muscle fibers and to actomyosin threads where less ambiguous data can decide whether both heads are necessary for production of tension.